One of the major open challenges for high confinement silicon photonic integrated circuits is to achieve polarization independence. Large polarization mode dispersion (“PMD”), polarization dependent loss (“PDL”), and polarization dependent wavelength characteristics are caused by structural birefringence in silicon strip waveguides. Polarization independent photonic integrated circuits may be achieved by using a silicon waveguide core that is exactly square in shape. In reality, however, fabrication errors of only a few nanometers would result in significant birefringence. Quantitatively, for example, given a waveguide cross section of 300 nm×300 nm, a variation of ±5 nm in width results in a differential group delay of 7 ps over a 5 cm length. Consequently, a 40 Gbps high-speed data stream would be significantly degraded under such conditions. Furthermore, the waveguide core width and height fluctuate randomly along the direction of light propagation due to fabrication tolerances. The random fluctuation varies the group index and results in polarization dependent wavelength characteristics of wavelength filters. Quantitatively, transverse-electric (“TE”) and transverse-magnetic (“TM”) modes exhibit a 100 GHz difference in resonance frequency in a 10 micrometer radius ring resonator with a 1 nm variation in waveguide width. Extremely challenging nanometer accuracy is therefore required for silicon strip waveguide devices used in polarization-independent dense wavelength division multiplexing systems.
To achieve polarization independent photonic integrated circuits, devices and architectures that attempt to rotate and control optical polarization have been pursued. These devices and architectures are referred to as systems with polarization diversity (or polarization transparency). These approaches include asymmetric gratings, waveguides with asymmetric slanted sidewalls, dual core waveguides with asymmetric axes, waveguides with asymmetric trenches, triple waveguide couplers, and bi-layer slots. Y. Yue et al., “Higher-order-mode assisted silicon-on-insulator 90 degree polarization rotator,” Optics Express 17, 20694-20699 (2009). S.-H. Kim et al., “Single-trench waveguide TE-TM mode converter,” Optics Express 17, 11267-11273 (2009). H. Fukuda et al., “Polarization rotator based on silicon wire waveguides,” Optics Express 16, 2628-2635 (2008). N.-N. Feng et al., “Low-loss compact-size slotted waveguide polarization rotator and transformer,” Optics Letters 32, 2131-2133 (2007). B. M. A. Rahman et al., “Design and Characterization of Compact Single-Section Passive Polarization Rotator,” Journal of Lightwave Technology 19, 512-(2001). P. Chan et al., “Mode conversion and birefringence adjustment by focused-ion-beam etching for slanted rib waveguide walls,” Optics Letters 28, 2109-2111 (2003). These approaches to realize polarization rotators in silicon suffer from several drawbacks. First, these approaches are static, in the sense that they rotate the polarization by only a fixed amount. Rotation angles vary among approaches and can be as low as 39°, corresponding to TE-TM conversion efficiencies of 40%. In addition, these approaches rely on asymmetric geometries with impedance mismatches resulting in degradation of insertion loss. Further, these approaches exhibit wavelength dependent loss because they rely on periodic structures or mode coupling.